The hotter it is, the slower you run - but humidity plays a role too. When the air is full of moisture, sweating doesn't cool you down like it usually would. You won't be able to run as fast, because your body is trying to prevent you from overheating.
Ever travelled overseas and tried to run through a wall of humidity? It feels like the air is considerably denser.
A chart from Runners Connect attempts to nail the effects of humidity down to a number. This is only approximate, since the way you overheat also depends on other factors like your body size and shape (the bigger you are, the easier it is to overheat) and whether you're already used to exercising in the heat.
To use the chart, you'll need to know the dew point. To find it, plug in today's temperature and per cent humidity into this dew point calculator. Then refer to the chart to see how much your running times will suffer.
For example, say your forecast is showing a high of 25 degrees Celsius with 85 per cent humidity. That's a dew point of 22 degrees. You should expect your times to be five to eight per cent longer than usual for the same distance. That means a run that takes 30 minutes on a good day will take around 32 minutes today. Or to put that in simpler terms, just look at the last two columns on the chart.
They say that easy running in this humidity will be "difficult" and hard running will be "very difficult." You may still choose to run outside, but at least you know what you're getting into.
How to Calculate the Effect of Humidity on Running Performance [Runners Connect]